The selective etching of semiconductor wafers by a plasma gas has been used by the semiconductor industry for many years. Much has been invested in the search for methods to control critical dimensions, as well as methods for determining when to stop etching. If etch time is over done the result can be poor control of dimensions relative to the target, missing geometries, poor performance, and the like. If etch time is too low the result can be oversized dimension of circuit elements, circuit shorts, open or highly resistive contacts, or other undesirable results. Control of the etch time is often simply a fixed time that has been determined by experimentation. The result is adequate but not optimum, especially not for a specific wafer. The relevant art includes methods for determining when to stop etching based upon various sensed parameters. This is difficult when the area being etched is small compared to the area protected by the masking material. The present invention provides a method for determining when to stop etching an instant wafer, even when the area to be etched is a small per cent of the whole surface.